The present invention relates to an optical disc apparatus which performs recording or reproduction while reducing a laser noise of a laser diode and a method for controlling the apparatus.
Recently, the digital versatile disc (hereinafter, referred to as DVD) attracts an attention as a high-density optical disc capable of recording large-volume digital information. However, as the information volume increases, the optical disc should have a higher density. Here, in order to achieve a higher density of recording than a DVD, it is necessary to record a mark smaller than the DVD on the recording layer. For this, the light source should have a short wavelength and the objective lens should have a large numerical aperture (hereinafter, referred to as NA). For example, in the case of DVD, a laser having wavelength of 660 nm is used as the light source, and a lens having NA of 0.6 is used as the objective lens. The Blu-ray Disc (hereinafter, referred to as BD) uses a laser having a wavelength of 405 nm as the light source and the objective lens having NA of 0.85, thereby achieving the recording capacity multiplied by 5 as compared to the DVD. Furthermore, recently, the blue laser has acquired a higher output and in order to obtain a higher recording capacity than a so-called single-layer disc having one recording layer, a multi-layer disc having a plurality of recording layers is being developed. For example, if a dual-layer recording disc is realized, the recording capacity can be 10 times higher than the DVD. However, in the conventional high-density optical disc apparatus, various storage margins upon reproduction are strict as compared to the DVD and it is impossible to ignore the affect of the quantization noise (quantization noise: noise generated when the laser diodes emits light) of the blue laser as the light source. Thus, as the recording density becomes higher and higher, the affect of the quantization noise generated by the laser diode upon light emission becomes larger. In order to solve this problem, there has been suggested to use an attenuator. Here, the attenuator is an element to attenuate a laser power (laser power: energy in w per unit time) of the light emitted from the laser diode. The attenuator may use, for example, an ND filter, a liquid crystal element, and a diffraction grating.
Use of an element such as an attenuator to attenuate the light intensity is disclosed, for example, in JP-A-2003-257072 ((Abstract), (0005) to (0018)) and JP-A-2004-199755 ((Abstract), (0014), (0072), (0073)).
JP-A-2003-257072 discloses a technique to arrange an optical element capable of changing light attenuation between a laser diode and an optical disc and changes the light attenuation factor in accordance with the type of the optical disc. Thus, even if the type of the optical disc is changed, the output power of the light source may be increased to a high power, which makes the quantization noise sufficiently low, thereby realizing an optical disc apparatus for reproducing radiation power onto the recording layer of the optical disc as a low power not causing degradation of the recording layer or erase of data.
JP-A-2004-199755 discloses a technique to arrange an optical element capable of varying the light attenuation factor between the laser diode and the optical disc so that the light is attenuated only during a low-power operation. Thus, during a low-power operation, it is possible to reduce the quantization noise and prevent a disc recording layer destruction and during a high-power operation, it is possible to avoid the laser power consumption increase and shortening of the service life.
Moreover, in general, when reproducing an optical disc, control is performed to maintain the laser power applied onto the optical disc surface to be constant. In the JP-A-2003-308623 ((Summary)), a monitor is arranged to measure a laser power immediately after being emitted from the laser so as to stabilize the laser output.